onyx pt, the mopar subwoofer comes with a 50 watt amp and 8" sub, it replaces your left rear trim panel. You should be able to install it yourself.
Here's a link to Wyckoff Chrysler site for the sub:
http://wyckoffchryslerparts.com/ptcruissub.html
You should check e-Bay also, I've seen them listed occassionally at a much better price.
Here's a link on installation of the MOPAR subwoofer:
http://www.ptdoityourself.net/ptsubintro.html